How do I convert a ASP.NET method to ASP.NET .asmx web service

I have following asp.net method which I want to convert it into .asmx web service. How I can do that. Any help please. public ActionResult AddCpty(string Id, string Type) { //code for Addcp return result; }

I am trying to convert it by putting a [WebMethod] attribute on the top of the method.

[WebMethod] public ActionResult AddCpty(string Id, string Type) { //code for Addcp return result; }